This invention pertains to microlithography apparatus for transferring a pattern (e.g., a circuit pattern), defined by a reticle, onto a sensitive substrate (e.g., semiconductor wafer) using a charged particle beam (e.g., electron beam or ion beam), as used in the manufacture of, e.g., semiconductor integrated circuits and displays.
Increases in the level of integration of semiconductor devices have so far kept pace with demand for increasingly more intricate integrated circuits. To meet this demand, it has been necessary that microlithographic exposure apparatus used in the manufacture of such devices be capable of resolving circuit features having increasingly smaller critical dimensions so as to produce such increasingly intricate circuits. In view of the resolution limits of optical microlithography, microlithographic apparatus employing a charged particle beam (e.g., an electron beam) are the subject of much interest as the candidate pattern-transfer technology for achieving resolution of pattern features that are substantially smaller than resolvable by optical microlithography.
In charged-particle-beam (CPB) microlithography, the pattern is usually defined by a reticle. The reticle is illuminated by the charged particle beam; charged particles in the beam passing through the illuminated portion of the reticle carry downstream of the reticle an image of the illuminated portion. The image-carrying beam is focused onto a corresponding region of the substrate which is coated with a suitable xe2x80x9cresistxe2x80x9d that imprints the pattern. Thus, the reticle pattern is xe2x80x9ctransferredxe2x80x9d to the substrate.
Due to various reasons, the entire reticle pattern is typically not illuminated at any one instant by the charged particle beam. Hence, the reticle field is typically divided (xe2x80x9csegmentedxe2x80x9d) into multiple exposure units such as xe2x80x9cstripes,xe2x80x9d xe2x80x9csubfields,xe2x80x9d or other regions (each defining a respective portion of the pattern) that are individually and sequentially exposed by the charged particle beam onto corresponding regions of the substrate. Projection of individual images on the surface of the substrate is accurately controlled to ensure that the images are contiguous with each other to form an image of the complete pattern without overlaps or intervening spaces between projected portions of the pattern. The resulting image of a complete reticle pattern on the substrate surface is termed a xe2x80x9cdiexe2x80x9d which is typically coextensive with one of multiple xe2x80x9cchipsxe2x80x9d typically exposed on the surface of a wafer. For details of this process, reference is made to Japanese Kxc3x4kai Patent Publication No. HEI 8-64522.
A schematic diagram of a conventional CPB microlithography apparatus is schematically illustrated in FIG. 1, depicting a charged particle beam (e.g., an electron beam), a crossover aperture 2a, a scattering aperture 2b, projection lenses 3a-3b, a substrate 5, an optical axis 6, a reticle 7, a CPB source 8, and condenser lenses 9a-9c. The reticle 7 is irradiated by the charged particle beam 1 emitted from the CPB source 8 via the condenser lenses 9a-9c. The charged particle beam 1 illuminates a selected region of the reticle 7; the beam passing through the illuminated region of the reticle 7 passes through the projection lenses 3a-3b and the scattering aperture 2b to form an image of the illuminated portion of the reticle on the upstream-facing surface of the substrate 5.
The combination of the condenser lenses 9a-9c (and the beam-shaping aperture 2a if present) is termed herein an xe2x80x9cillumination-optical system.xe2x80x9d Similarly, the combination of the projection lenses 3a-3b (and the scattering aperture 2b if present) is termed herein a xe2x80x9cprojection-optical system.xe2x80x9d Each of the lenses 9a-9c, and each of the lenses 3a-3b, can be either an electromagnetic lens or an electrostatic lens as known in the art.
In conventional CPB microlithographic exposure, the charged particle beam is focused by the CPB projection-optical system comprising the projection lenses 3a-3b. Depending upon the specific type of projection-optical system used to make the exposure, if the projection lenses 3a, 3b are radially symmetrical about the optical axis 6 (as they typically are), then significant spherical aberrations are usually generated. Spherical aberrations tend to cause blurring of the image as projected onto the substrate.
Whenever edges of features of the pattern as projected are blurred, the resulting pattern has poorly transferred features with, for example, dimensions that are outside specification. Such exposure problems can result in unacceptable deviations in electrical performance of the circuit during use of the semiconductor device, such as breaks, shorts, high-resistance loci, poor gate performance, and the like, which ultimately result in failure of the semiconductor device to meet specification. Consequently, it is necessary to reduce spherical aberration as much as possible.
Spherical aberration is proportional to the cube of the xe2x80x9cbeam semi-anglexe2x80x9d xcex1. The beam semi-angle can be measured at the substrate or at any of various other locations, especially where the beam is convergent or divergent, such as at stop locations. The beam semi-angle is the angular divergence of the charged particle beam from the optical axis at the measurement location. Denoting Csph as a spherical-aberration coefficient, Csphxcex13 expresses the magnitude of the corresponding spherical aberration. Consequently, reducing the beam semi-angle at the image plane provides a way in which to decrease spherical aberration.
According to one proposed approach, multi-pole projection lenses are used. However, whereas aberration-correction methods that utilize multi-pole lenses are theoretically appealing, such methods exhibit considerable problems because manufacturing multi-pole lenses is extremely difficult. Furthermore, aligning the optical axes of multi-pole lenses is very difficult.
Aberration correction using multi-pole lenses has not yet become practical also because actual lens-field distributions exhibit considerable deviation from design specifications as a result of manufacturing errors and alignment errors in such lenses. Such deviations typically result in additional aberrations that degrade image fidelity and resolution. Okayama, Electron Microscope 25(3):159-166, 1990. Another practical problem is that operation of multi-pole lens systems requires extremely complex electrical control systems.
Besides lens aberrations, blurring of the charged particle beam can result from a xe2x80x9cCoulomb effectxe2x80x9d which is a Coulombic repulsion occurring between charged particles propagating near each other in the beam, especially at points of convergence and divergence. As the beam semi-angle is reduced, the particles in the beam tend to propagate closer together, thereby increasing the Coulomb effect. By increasing the beam semi-angle, the diameter of the electron beam is increased at points of convergence and divergence (e.g., stop locations), which results in an increased spacing between charged particles at such locations. This increased spacing weakens Coulombic repulsion in inverse proportion to the square of the distance between the particles, thereby resulting in less image blurring. Such a situation is illustrated in FIG. 2, in which the abscissa is the beam semi-angle and the ordinate is the magnitude of blurring.
Unfortunately, geometric aberrations (including spherical aberration) exhibited by lenses and deflectors used to focus and deflect the charged particle beam tend to increase as the beam semi-angle increases. Such increases in geometric aberrations tend to increase beam blurring, as also shown in FIG. 2.
With a multi-pole lens system, the interaction time between adjacent charged particles propagating in a beam increases because the distance from the object plane to the image plane is correspondingly increased. Such increased interaction time also increases beam blurring due to the Coulomb effect.
In view of the above, a practical way in which to effectively reduce blurring due both to the Coulomb effect and to spherical aberrations has not yet been realized.
Referring further to FIG. 1, according to conventional wisdom, it is preferable that the electron beam 1 have as high a beam current as possible so as to increase xe2x80x9cthroughputxe2x80x9d (processing capacity per unit time). However, there are practical limits to the magnitude of the beam current. Namely, whenever the beam current is high, random scattering tends to occur between adjacent or nearby charged particles in the beam due to Coulombic repulsion. As a result, the beam becomes blurred by the time it reaches the substrate 5.
Another variable that must be considered with respect to image quality is the xe2x80x9cspace-charge effect.xe2x80x9d A charged particle beam generates its own electrical field. The space-charge effect acts on each individual charged particle in the beam propagating through that self-generated electrical field. The space-charge effect affects lens operation by changing the convergence of the beam in a manner similar to a normal electrostatic lens employing an electrostatic field to shape the beam. Such changes to beam convergence induces aberrations that are similar to lens aberrations, such as field curvature and distortion. The space-charge effect can also cause, independently of blurring due to random scattering, decreased resolution of the image produced by the charged particle beam.
In addition, the magnitude of the beam current passing through the reticle varies from region to region (e.g., subfield to subfield) depending upon normal differences in the feature density from region to region (e.g., subfield to subfield). Such changes in beam current manifest themselves as lens effects. Hence, even when the beam current illuminating the reticle 7 is maintained constant, the focal point of the beam will differ from region to region (e.g., subfield to subfield) as a function of region-mediated changes in beam current downstream of the reticle. This situation is shown in FIGS. 3(a)-3(b), in which item 5 is the substrate and item 3b is the second projection lens. In FIG. 3(a), the focal point is coincident with the substrate surface by action of the projection lens 3b on a relatively low-current beam.
When the beam current is relatively higher, as shown in FIG. 3(b), the focal point is not coincident with the substrate surface. In such an instance, correction is required. Since the individually exposed regions (e.g., subfields) on a reticle 7 typically have a variety of respective feature densities, the corresponding beam current will typically be different from region to region (e.g., subfield to subfield), thereby requiring correction for each individual region (e.g., subfield).
The higher the beam current used to illuminate the reticle 7, the wider the range of focus correction required. Whatever the correction system selected, it must be capable of operating at high speed over such a range of focal-point variation. Furthermore, the wider the range of correction required, the more problematic the aberrations generated by any focus-correction system that is employed, and the more severe the burden placed on an electrical control system for such a focus- correction system to ensure satisfactory operation at high speed over the range.
The beam semi-angle of a charged particle beam used for microlithography cannot be made too large due to limitations imposed by geometric aberrations in the lenses and/or deflectors. Therefore, there is a practical limit to the beam current. As a result, it has been difficult to obtain both high throughput and high resolution using high-current charged particle beams in conventional CPB microlithography apparatus.
The shortcomings of the prior art as summarized above are corrected by apparatus and methods according to the present invention.
According to a first aspect of the invention, charged-particle-beam (CPB) microlithography apparatus are provided that exhibit much less spherical aberrations and other aberrations. A first embodiment generally comprises an optical axis, an illumination-optical system, a projection-optical system, and an xe2x80x9caperture plate.xe2x80x9d The illumination-optical system is configured and situated so as to direct a charged particle beam from a CPB source to a reticle and illuminate a region of the reticle with the beam. The projection-optical system is configured and situated relative to the reticle so as to direct the charged particle beam, after the beam passes through the illuminated region of the reticle, to a substrate so as to imprint an image of the illuminated region on a corresponding exposure region on the substrate. The aperture plate is situated upstream of the substrate, and defines an aperture therethrough that transmits a portion of the charged particle beam. The transmitted portion, when incident on the substrate, has a beam semi-angle that is greater than a lower limit that is not zero, and less than an upper limit greater than the lower limit. The aperture plate can be situated, for example, within the illumination-optical system or the projection-optical system. Within such regions the aperture plate can be situated within a region in which the charged particle beam is collimated.
The aperture defined by the aperture plate is preferably annular in general profile or at least functions as if the aperture were annular in general profile. The aperture is preferably centered on the optical axis. (As an alternative to an aperture plate as summarized above, an annular cathode can be used.)
The apparatus generally exhibits a spherical aberration that, as a result of paraxial portions of the beam being blocked by the aperture plate, is lower in magnitude than spherical aberration exhibited by an otherwise similar charged-particle-beam microlithography apparatus lacking the aperture plate.
The apparatus can include a mechanism for moving the substrate along the optical axis to place the substrate at an axial position at which blurring of the charged particle beam incident on the substrate is maximally reduced. Alternatively, the apparatus can include a corrective lens that refracts the charged particle beam before the beam is incident on the substrate. Such refraction is preferably sufficient to maximally reduce blurring of the charged particle beam as incident on the substrate.
The charged particle beam can consist of any of various charged particles such as electrons and/or ions. If the beam is an electron beam, the projection-optical system preferably satisfies the conditions:
ALa(Iillum/4)b/[Vc(xcex1wafer-max)d]xe2x89xa640xe2x88x922.5(xcex1wafer-maxxe2x88x9210)
Iillum24 70 xcexcA
Vxe2x89xa6200 KeV
Lxe2x89xa7300 mm
wherein Iillum is the beam current (in xcexcA) of the electron beam as incident on the reticle, L is the axial distance (in mm) between the reticle and the substrate, V is the beam-acceleration voltage (in KeV) of the beam incident on the reticle, xcex1wafer-max is the maximum value of the beam semi-angle of the beam as incident on the substrate, and A, a, b, c, and d are constants, wherein 61xe2x89xa6Axe2x89xa681, 1.2xe2x89xa6axe2x89xa61.4, 0.6xe2x89xa6bxe2x89xa60.85, 1.3xe2x89xa6cxe2x89xa61.6, and 0.6xe2x89xa6dxe2x89xa60.8. Preferably, xcex1wafer-max=mxcex1mask-max, wherein 1/m is the demagnification ratio of the projection-optical system.
According to another embodiment, a CPB microlithography apparatus is provided that comprises, on an optical axis, an illumination-optical system, a projection-optical system, and an aperture plate. The illumination-optical system is configured and situated so as to direct a charged particle beam from a CPB source to the reticle and illuminate a region of the reticle with the charged particle beam. The projection-optical system is configured and situated relative to the reticle so as to direct the charged particle beam, after having passed through the illuminated region of the reticle, to the substrate so as to imprint an image of the illuminated region on a corresponding exposure region on the substrate. The aperture plate is situated upstream of the substrate and comprises a central portion situated so as to block paraxial portions of the beam smaller than a first diameter and a peripheral portion situated so as to block off-axis portions of the beam greater than a second diameter that is larger than the first diameter. The aperture plate provides the beam as incident on the substrate with a range of beam semi-angles not including a zero beam semi-angle. If the beam is an electron beam, then the projection-optical system preferably satisfies the conditions:
ALxcex1(Iillum/4)b[Vc(xcex1wafer-max)d] less than 40xe2x88x922.5(xcex1wafer-maxxe2x88x9210)
Iillumxe2x89xa770 xcexcA
Vxe2x89xa6200 KeV
Lxe2x89xa7300 mm
xe2x80x83wherein Iillum, L, V, xcex1wafer-max, A, a, b, c, and d are defined above.
According to another aspect of the invention, methods are provided for performing microlithographic projection-transfer of a pattern, defined by a reticle, onto a sensitive surface of a substrate. A charged particle beam is directed from a CPB source to the reticle to illuminate a region of the reticle with the charged particle beam. The beam passing through the illuminated region of the reticle is directed to the sensitive surface of the substrate so as to imprint an image of the illuminated region on a corresponding exposure region on the sensitive surface. Beam blurring at the sensitive surface as caused by any one or more of geometrical aberrations (which include spherical aberrations), Coulomb effects, and space-charge effects is reduced. Such reduction is achieved by, e.g., blocking paraxial portions of the beam smaller than a first diameter, and blocking off-axis portions of the beam greater than a second diameter that is larger than the first diameter. Thus, portions of the charged particle beam having a beam semi-angle that is greater than a lower limit greater than zero, and less than an upper limit greater than the lower limit, are allowed to pass to the exposure region on the substrate.
The method can further include detecting the axial position of the optimal image plane of the beam. The substrate can be moved along the axis as required to place the sensitive surface at the optimal image plane.
In an alternative embodiment, paraxial portions of the beam smaller than a first diameter are blocked. In addition, off-axis portions of the beam are blocked that are greater than a second diameter larger than the first diameter. Thus, portions of the charged particle beam having a beam semi-angle within a limited range not including a zero beam semi-angle are allowed to propagate to the sensitive surface. The method can include detecting the axial position of the optimal image plane of the beam. The beam is refracted and focused for forming an image of the illuminated region on the sensitive surface, and the optimal plane is adjusted to the sensitive surface. If the beam is an electron beam, the beam is preferably provided with a range of beam semi-angles in which the minimum beam semi-angle (xcex1mask-min) at the reticle and the maximum beam semi-angle (xcex1mask-max) at the reticle are 1.5 to 3.0 mrad. Further preferably, |xcex1mask-minxe2x88x92xcex1mask-max|xe2x89xa60.75 mrad.
According to another aspect of the invention, CPB microlithography apparatus are provided that comprise, along an optical axis, an illumination-optical system, and a projection-optical system. As in other embodiments summarized above, the illumination-optical system is configured and situated so as to direct the charged particle beam from a source to the reticle and illuminate a region of the reticle with the beam. The projection-optical system is configured and situated relative to the reticle so as to direct the beam, passing through the illuminated region of the reticle, to the substrate so as to imprint an image of the illuminated region on a corresponding exposure region on the substrate. The projection-optical system also comprises a scattering aperture concentric with the optical axis. A first group of at least six deflectors (for aberration correction) is situated between the reticle and the scattering aperture, and a second group of at least three deflectors (also for aberration correction) is situated between the scattering aperture and the substrate. The deflectors in the first group and the deflectors in the second group can be either electromagnetic deflectors (energized by respective electrical currents applied to the deflectors) or electrostatic deflectors (energized by respective electrical voltages applied to the deflectors). The deflectors of the first and second groups are preferably independently energizable so as to cause the beam to assume a trajectory in which deflection-induced aberrations are corrected, such as exposure-region image plane inclination, deflection coma, exposure-region astigmatism, chromatic aberration, and exposure-region secondary distortion. In any event, the beam is caused to pass centrally through the scattering aperture to a target exposure region on the substrate, at which exposure region the beam has a zero angle of incidence.
The apparatus can include a device, disposed upstream of the substrate or upstream of the reticle, for limiting the beam semi-angle of the charged particle beam incident on the substrate to within a range greater than a lower limit that is not zero and an upper limit greater than the lower limit. Such a device can be, for example, an aperture plate defining an annular aperture. If the device is located upstream of the reticle and if the beam is an electron beam, then the lower limit (xcex1mask-min) is preferably 1.5 mrad and the upper limit (xcex1mask-max) is preferably 3 mrad. Also, |xcex1mask-minxe2x88x92xcex1mask-max| is preferably less than or equal to 0.75 mrad. Furthermore, if the beam is an electron beam, the projection-optical system preferably satisfies the conditions:
ALxcex1(Iillum/4)b/[Vc(xcex1wafer-max)d]xe2x89xa640xe2x88x922.5(xcex1wafer-maxxe2x88x9210)
Iillumxe2x89xa770 xcexcA
Vxe2x89xa6200 KeV
Lxe2x89xa7300 mm
xe2x80x83wherein Iillum, L, V, xcex1wafer-max, A, a, b, c, and d are defined above. By satisfying these expressions, blurring due to random scattering caused by Coulombic repulsion is made dependent on the value of xcex1wafer-max, and can be decreased to around 35 to 50 nm.
Apparatus and methods according to the invention are especially capable of achieving pattern resolutions of less than 100 nm, including resolutions of, e.g., 90 nm or less. Throughput is not compromised because a high-current beam can be used.
Apparatus and methods according to the invention are also especially suitable for projection-transfer of patterns defined on a segmented reticle on which the pattern is divided into exposure regions such as subfields or stripes.
The limit value xcex1mask-min is preferably 1.5 mrad or more for an electron beam because, if the value were less than 1.5 mrad, the percentage of electrons at the center of the angular distribution would be too high to achieve the desired effect of annular illumination. This, in turn, would increase blurring due to space-charge effects and would make it difficult to achieve the target resolution of 90 nm. The limit value amask-max is preferably 3 mrad or less because, if the value were greater than 3 mrad, then geometric aberrations generated by the projection-optical system would be too great to achieve the target resolution of 90 nm (even if the Coulomb effect can be adequately suppressed).
|xcex1mask-minxe2x88x92xcex1mask-max| was limited to 0.75 mrad or less because, if this value were exceeded, the annular illumination of the reticle would be too wide, thereby diminishing the beneficial effects of annular illumination. This, in turn, would increase blurring due to space-charge effects and make it difficult to achieve the target resolution of 90 nm.
With an electron beam, the beam current (Iillum) as incident to the reticle surface is preferably set at 70 xcexcA or greater to provide sufficient beam current for acceptable throughput. It is also preferable that the beam-acceleration voltage (V) be high to reduce blurring due to Coulombic repulsion. However, the sensitivity of electron-beam resists drops radically and the processing capacity is too low when the beam-acceleration voltage V exceeds 200 keV. Therefore, the beam current for an electron beam is preferably no greater than 200 keV.
In addition, blurring is acceptably low if the distance L between the mask and the substrate is minimal. However, for electron-beam systems, it is preferred that the distance L be at least 300 mm to avoid problems due to excessive electrical current being supplied to the lenses in the illumination-optical system and projection-optical system (which excessive current leading to problems with excessive heating).
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.